A critical component of the acute inflammatory response is the rapid mobilization of blood- borne lymphocytes into secondary lymphoid organs. These organs are the staging ground for lymphocyte encounters with antigens and foreign pathogens during the initiation of protective immunity. Significant progress has been achieved in defining the adhesion events that guide homeostatic steady-state trafficking of lymphocytes across vascular checkpoints in lymphoid organs. By contrast, the molecular basis of inducible trafficking of naive and central memory cells to lymphoid organs during inflammation is poorly understood. Extensive preliminary data lead us to hypothesize that the proinflammatory cytokine, interleukin-6 (IL-6), is a driving force in regulating lymphocyte migration into lymphoid organs during acute inflammation. The first aim will identify the cellular source of IL-6 that regulates the capture efficiency of vascular gateways in a model of systemic febrile inflammation. Reciprocal bone marrow chimeras with wild-type and IL-6-deficient mice will segregate whether IL-6 production by radiation-resistant stromal cells or radiation-sensitive hematopoietic cells is required for enhanced lymphocyte trafficking across vessel walls during febrile stress. Homing assays and intravital microscopy will further validate that a defined cellular source of IL-6 promotes lymphocyte influx into lymphoid organs. Aim 2 will focus on determining if IL-6 is also responsible for mobilizing the recruitment of naive cells to local inflamed lymph nodes during an adaptive immune response. These studies are based on our surprising discovery that IL-6 produced by mature dendritic cells modifies the adhesive properties of vascular entryways. The final aim will use genetic approaches to dissect the IL-6 downstream signal transduction pathways that regulate lymphocyte trafficking during local and systemic adaptive immune responses. The studies will use mutant mouse lines that have specific defects in IL-6 signaling pathways in order to map the molecular mechanisms required for accelerated lymphocyte trafficking during acute inflammation. Understanding the cytokine requirements for lymphocyte recruitment during acute inflammation may lead to novel intervention strategies in chronic inflammatory disorders as well as provide insights into vaccine approaches based on the ability of IL-6 to heighten adaptive immunity.